Interaction of barbiturates with benzodiazepine receptors in the central nervous system

The interaction of barbiturates with benzodiazepine receptors was studied in extensively washed membrane preparations from rat brain. Sedative/hypnotic and anesthetic barbiturates such as pentobarbital, and convulsant barbiturates such as DMBB, enhanced [3H]diazepam binding in a stereospecific fashion. Freeze-thawing of membranes resulted in a decrease in the potency of barbiturates to enhance [3H]diazepam binding, while the maximum response to barbiturates remained unchanged. Significant differences in both the potency and maximum enhancement of [3H]diazepam binding by pentobarbital was observed among brain regions. The rank order potency of pentobarbital in different brain regions was: cerebellum greater than cortex greater than hippocampus, while the rank order efficacy of pentobarbital in these brain regions was reversed. The effects of a combination of anesthetic and/or convulsant barbiturates on [3H]diazepam binding suggested that these compounds function as partial agonists while a combination of anesthetic or convulsant barbiturates with phenobarbital suggested that latter compound antagonized the actions of both anesthetic and convulsant barbiturates. The convulsant benzodiazepine Ro-5-3663 and inosine were more potent as inhibitors of pentobarbital-enhanced than basal (non-pentobarbital enhanced) [3H]diazepam binding. Solubilization of benzodiazepine receptors with Lubrol-PX resulted in a complete loss of barbiturate enhanced [3H]diazepam binding, and greater than a 75% loss in efficacy in the remaining (insoluble receptor) tissue. These data, coupled with recent observations from this and other laboratories, suggests that the site(s) at which barbiturates act to enhance [3H]diazepam binding to benzodiazepine receptors is distinct from the site at which GABA acts to enhance [3H]diazepam binding. The phenomenon of enhanced benzodiazepine binding by barbiturates may be related to the depressant actions of the barbiturates, that is, their direct effects to increase chloride conductance. Although it is premature to assign a pharmacologic correlate to this neurochemical phenomenon, it appears that this action may be related to the anesthetic effects of the barbiturates. However, the definitive assignment of either the electrophysiologic or pharmacologic sequelae to this neurochemical action will require further investigation.